1. Field of the Invention
Aspects of the present invention relate to an organic light emitting display, and more particularly, to an organic light emitting display including a thin film transistor (TFT) in which an N-type oxide semiconductor layer is formed.
2. Description of the Related Art
In a TFT of a conventional organic light emitting display, amorphous silicon or poly silicon is used as a semiconductor layer. However, when the semiconductor layer is formed of amorphous silicon, it is difficult to use the TFT as a driving circuit of a display panel that requires high operating speed because of low mobility in the amorphous silicon layer When the semiconductor layer is formed of poly silicon, mobility is high, but the threshold voltage is non-uniform. Thus, an additional compensating circuit is necessary. Moreover, a leakage current is generated by the TFT having a semiconductor layer of amorphous silicon or poly silicon because of the radiation of light so that the characteristics of the TFT deteriorate.
Therefore, oxide semiconductors are being studied to solve such problems. For example, Japanese Patent Publication No. 2004-273614 discloses a TFT that has a semiconductor layer formed of ZnO or an oxide semiconductor including ZnO. Referring to FIG. 1, the TFT includes a source electrode 20a and a drain electrode 20b formed on an insulating substrate S, a ZnO layer 4 provided to contact the source and drain electrodes 20a and 20b, and a gate insulating layer 5 and a gate electrode 6 laminated on the ZnO layer 4.
In this case, the ZnO or the oxide semiconductor including ZnO has a band gap of 3.4 eV, which is higher than light energy in the visible light region, and thus the ZnO does not absorb visible rays. Therefore, in the TFT, the leakage current caused by the absorption of visible light does not increase. However, it is generally the case that ZnO or an oxide semiconductor including ZnO exhibits N-type conductivity caused by the depletion of O2, whereas an organic light emitting display uses a P-type driving element.
In another example, Korean Patent Registration No. 635514 discloses an organic light emitting display using a common P-type driving element. Referring to FIG. 2, a P-type driving TFT includes a gate electrode 40, a source electrode 50a, a drain electrode 50b, and a P-type semiconductor layer 60 formed on a substrate 30.
In this case, the source electrode 50a of the driving TFT is coupled to a common power source voltage line ELVDD 96, and the drain electrode 50b is coupled to the anode electrode 80 of an organic light emitting diode (OLED). An organic light emitting layer 90 is formed on the anode electrode 80, and a cathode electrode 92 formed on the organic light emitting layer 90 is coupled to a second power source voltage line 97, so that the organic light emitting display is driven. Reference numerals 93, 94, 98, and 99 that are not described further herein denote an insulating layer, a pixel defining layer, a sealing material, and an encapsulating substrate.
In order to use a driving TFT in which an N-type oxide semiconductor layer is used for an organic light emitting display, the structure of the anode electrode of the OLED and the structure of the cathode electrode thereof should be reversed. Therefore, it is necessary to improve the structure of an organic light emitting display, and thus studies are being carried out to improve the structure of organic light emitting displays.